1. Field of the Invention
The present invention relates to means for applying power to an AC load at the crest of an alternating voltage waveform to thereby prevent current surges during activation of the AC load and is more particularly concerned with means for detecting a half-cycle voltage crest of the alternating voltage waveform.
Generally speaking, the present invention is an improvement upon means for detecting a half-cycle voltage crest of an alternating voltage waveform where the improvement includes a means for protecting the detecting means from current surges associated with the initial random application of the alternating voltage waveform to the detecting means.
2. Description of the Prior Art
With certain types of AC loads and particularly with those requiring a substantial amount of power for initial activation if not activated at the crest of an alternating voltage waveform such as the primary winding of a transformer used to drive a magnetron power supply in a microwave oven, a means must be employed to limit current surges during start-up; otherwise, fuses and circuit breakers used to prevent excessive start up current flow would be continuously thrown open as a result of excessive current being drawn by the load or in the absence of a fuse, circuit breaker, or other means for preventing excessive current flow, wiring and/or the AC load itself may be damaged. One method of preventing such surges is by activation of the load at the crest or peak voltage of the alternating voltage waveform. Alternatively, rather than attempt to have the load activate or draw power for activation at the crest of the alternating voltage waveform, a current limiting device such as a resistance means will be electrically coupled in series with the load to prevent current surges. However, the larger the value of the resistance means, the smaller the current available to provide the necessary power to operate the load. An additional switching device may be used to shunt the resistance means and apply full power to operate the load.
The present invention provides a means for detecting a half-cycle voltage crest of the alternating voltage waveform so that power may be applied to the AC load at the voltage crest thereby minimizing or eliminating current surges during activation of the load. Most prior art peak detector circuits are very sensitive to component values of the circuit and/or require complex logic circuitry to control the on and off switching of an AC load as a result of their inability to completely assure that all enabling signals will be provided at voltage crests. To overcome these problems, a means for detecting a half-cycle voltage crest of an alternating voltage waveform is disclosed in U.S. patent application, Ser. No. 865,309 entitled "Variable Power and Temperature Control System For Appliances And The Like" (See FIG. 1 of the present disclosure) which employs a capacitor having a discharge time constant which is substantially longer than a period of the alternating voltage waveform and a transistor for applying the alternating voltage waveform to the capacitor at substantially the half-cycle voltage crest of the alternating voltage waveform which also produces a signal indicating the voltage crest when applying the alternating voltage waveform. By this means a "window" of time extending from just before to just after the peak of the AC line voltage is formed during which power may be applied to the AC load.
Although the circuit disclosed in U.S. application, Ser. No. 865,309 alleviates the current surge problem during the initial activation of the AC load, if the AC line voltage (alternating voltage waveform) is suddenly applied to the circuit disclosed in U.S. application, Ser. No. 865,309 at a random phase of the alternating voltage waveform a current surge will be applied to the base of the transistor thereby damaging the transistor. Accordingly, a current surge problem still remains during the activation of the means which was devised to eliminate current surges during the activation of the AC load. By improving upon the design of the prior art circuit shown in FIG. 1, the remaining current surge problem can be alleviated.